Yes, you can get pregnant on your period especially if you have an irregular cycle

• Most people experience irregular cycles that make it difficult to track when you are going to ovulate next to prevent pregnancy.
• If you experience an extremely short cycle one month, then the time you ovulate could overlap with your period and you could get pregnant if you have unprotected sex around that time.
• Tracking your period and ovulation is one way to prevent pregnancy, but the most effective way is to use some method of birth control.
• This article was medically reviewed by Jamie Lipeles, DO, OB/GYN and founder of Marina OB/GYN in Marina Del Rey, California.
• Visit Insider's Health Reference library for more advice.

Though unlikely, it is possible to get pregnant while on your period. It's, "all a matter of timing," says Anna Childson, an OB/GYN for Kaiser Permanente in Falls Church, Virginia. "It has to be the right set of circumstances for it to happen." 

Here's what those circumstances might be.

When you are most fertile 

You are most fertile three days prior to ovulation, during ovulation, and two to three days afterward.

Once you ovulate, your system flushes the egg out within 12-24 hours if it's not fertilized. And you will not be able to get pregnant until around the next time you ovulate.

The first day of your period marks the start of your cycle, but most people don't ovulate until the middle of a menstrual cycle.

However, cycles vary in length for each person, which means the fertility window may happen earlier or later. 

Irregular periods increase the risk of a surprise pregnancy

For women with regular cycles, it's easy to track when you will ovulate to prevent pregnancy. However, many people experience irregular cycles, which makes it more difficult to know when you'll start your period and when you'll ovulate. 

"There are women who say, 'I get my period regularly every 35 days,' but it is much more common to hear women say, 'I never know when I'm going to get my period,'" says Joshua Copel, a professor of obstetrics, gynecology and reproductive services at the Yale School of Medicine. 

When you can get pregnant on your period

You can get pregnant while on your period if you happen to ovulate early during a short cycle. For example, someone with a cycle of 24 days might ovulate as early as day seven. So, if your period lasts the full week ending on day seven and you have sex during that time, you could get pregnant.

The likelihood of ovulating close to your period is low, but not impossible. A 2013 study in Human Reproduction estimated the date of conception for 5,830 pregnancies. More than half, about 58%, of people conceived around day 12 or 13 of their cycle. However, 2% of participants were within their fertility window on day four, which would almost certainly overlap with their period. 

Moreover, the length of a cycle is extremely broad. For example, a review, published in 2019 in Digital Medicine, analyzed more than 81,600 cycles and found that only 13% were 28 days long. Some were as short as 15 days, and others as long as 50.

Factors that change the time you ovulate

The sex hormones estrogen and progesterone largely determine the duration of a menstrual cycle. For example, over the course of a cycle, rising estrogen levels trigger ovulation and thicken the uterine lining so that you can potentially conceive and nurture a fertilized egg.

Slight changes in estrogen and progesterone levels can lead to changes in cycle duration and when you ovulate. These changes can come from factors like age, ethnicity, and body mass index.

Stress, illness, and other medical conditions can also affect when you ovulate, and alter the typical length of your cycle. "Even somebody with a 31-day cycle might have a shorter cycle than their average and have a slip-up," Childson says. 

Tracking your period can help you understand your cycle

Tracking your cycle may give you a better idea of how long your periods usually last and when you will ovulate. For example, some apps track symptoms like changing body temperature or cervical mucus that can help you pinpoint when you are ovulating in any given month. 

Another possible sign of ovulation is mid-cycle spotting which can occur when estrogen levels dip. However, there are a number of other reasons for why you might bleed in between periods including the following: 

• Birth control. Hormonal birth control can lead to mid-cycle breakthrough bleeding, especially when you're using a new prescription or missed a dose.
• An infected or inflamed cervix. Sexually transmitted infections as well as chemicals from a douche, spermicide, or tampon can irritate the cervix and cause bleeding. 
• Implantation. If an egg is fertilized, it will attach itself to the endometrium within a week, causing light spotting.   

Tracking your cycle carefully can help you recognize when bleeding between periods is normal for you, or is a reason for you to talk with your doctor. You'll also be able to determine whether or not it's actually your period. 

"There are many women who think that any vaginal bleeding is menstruation," Copel says.  A risky assumption if they have unprotected sex because it could lead them to incorrectly estimate when their fertility window falls. 

The best protection against pregnancy is birth control

Tracking your cycle — also known as the rhythm method — isn't a surefire way to avoid pregnancy. "The rhythm method is not terribly reliable for a number of reasons, including variability in timing of ovulation, in recording the first day of the last menstrual period, and in the longevity of both sperm and ova," Copel says. 

The best way to prevent becoming pregnant at any time during your cycle, short of abstinence, is to use birth control. 

Hormonal birth control, like pills, patches, and contraceptive rings, work by changing hormone levels to stop ovulation and make it harder for sperm to reach the egg and fertilize it These birth control methods are very effective at preventing pregnancy, with success rates ranging from 93% for birth control pills to 99% for arm implants.

Moreover, "I recommend condom use for anybody in a non-monogamous relationship," Childson says. Male and female condoms are less effective — 87% and 79% respectively — but they have the added benefit of preventing STIs like hepatitis, HIV, gonorrhea, and syphilis.

Related articles from Health Reference:

• How many periods you can miss before you should worry
• Yes, you can safely swim while on your period with or without a tampon
• Yes, birth control helps with cramps but some methods are better than others
• You can get pregnant with an IUD but it's extremely rare

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